Computer storage devices contain semiconductor devices such as DRAM. Recently, the capacity of a storage device becomes larger and larger and much effort has been made to develop the technology for increasing the capacity.
The technology for mounting semiconductor devices on both sides of a board is effective for increasing the capacity of a storage device. In addition, mounting a semiconductor device with a normal external terminal arrangement (hereinafter termed a normal package) on one side of a board and mounting a semiconductor device with an arrangement where the external terminals are arranged laterally or vertically symmetrical to those of a normal package (hereinafter termed a mirror package) in the corresponding position of the other side of the board increase the wiring efficiency of the board. This is because mounting a normal package and a mirror package in the corresponding positions on the both sides of a board allows their pins to be directly connected through via holes. This eliminates the need for creating patterns on the board. Similarly, in a semiconductor module including a plurality of semiconductor devices, mounting a normal package mounted on one side of a board with a mirror package on the other would increase wiring efficiency. In the present specification, external pins are laterally symmetrical, if they are arranged symmetrically or bilaterally in left and light sides of a package and external pins are vertically symmetrical, if they are arranged symmetrically in upper and lower sides of the package.
Conventionally, a mirror package is generally manufactured during the assembly process by replacing a wire between an external terminal (hereinafter termed a package pin or simply pin) and a semiconductor chip pad (hereinafter termed chip pads or simply pads) or by replacing a bonding wire connected to the chip pad. Recently, however, it is required that a normal package be changed to a mirror package after the assembly process.
Japanese Patent Kokai Publication JP-A-7-288282 discloses a semiconductor device with a pad switching circuit that selectively changes the pad functions to allow the pins to be switched after the assembly process. According to the semiconductor device disclosed in Japanese Patent Kokai Publication JP-A-7-288282, the signal pins are switched by the pad switching circuit, and the power supply voltage pin and ground pin are switched by reinstalling the bonding wires, to build a mirror package. This increases the wiring efficiency of the board.